In general, electrolyte solutions used in batteries and other electrochemical devices preferably have the property of resistance to oxidation and reduction. In other words, they are preferably electrolyte solutions with broad potential windows. For example, electrolyte solutions used in lithium secondary batteries have conventionally comprised lithium salts (supporting salts) dissolved in carbonate solvents such as ethylene carbonate, propylene carbonate, diethyl carbonate and the like, but from the standpoint of improving the performance of lithium secondary batteries, electrolyte solutions using solvents that are more resistant to oxidation than carbonate solvents would be preferable.
Boron trifluoride (BF3) exhibits strong acidity because the boron atom is bound to three strongly electron-withdrawing fluorine atoms. In a BF3 complex comprising an organic molecule coordinated with the unoccupied orbital of the boron in BF3, it is expected that the oxidation resistance of the organic molecule will be improved because electrons of the coordinated organic molecule are drawn towards the boron atom due to the strong electron-withdrawing properties of the BF3 part of the complex. Japanese Patent Application Laid-open No. 2008-94825 (Patent Document 1) describes an electrolyte solution containing a BF3 complex comprising a chain ether such as dimethoxyethane coordinated with BF3. In Japanese Patent Applications Laid-open Nos. H11-149943 and 2000-138072 (Patent Documents 2 & 3), BF3 complexes are only used in small amounts as additives to electrolytes, and are not themselves used as electrolytes. Japanese Patent Application Laid-open No. H02-262270 (Patent Document 4) describes a battery having as a component an organic electrolyte solution comprising BF3 or another compound having a B—F bond added to an organic electrolyte solution obtained by dissolving a lithium salt in an organic solvent containing a cyclic ether, but this electrolyte solution does not use a complex of BF3 or the like with an ether. Patent Document 4 describes the possibility of solvation between the BF3 or the like and the ether, but there is an obvious difference between solvation and complex formation.    Patent Document 1: Japanese Patent Application Laid-open No. 2008-94825    Patent Document 2: Japanese Patent Application Laid-open No. H11-149943    Patent Document 3: Japanese Patent Application Laid-open No. 2000-138072    Patent Document 4: Japanese Patent Application Laid-open No. H02-262270